Abstract T cell exhaustion is characterized by a progressive loss of inflammatory cytokines production followed by a functional and proliferation defect leading to apoptosis. While this phenomenon was studied recently in the context of allo-transplantation, it remains not well understood. Identifying and targeting key pathways involved in the induction of allo-immune T cell exhaustion will promote allograft acceptance. We recently showed that the immunoproteasome (i-20S) replaces the constitutional proteasome (c-20S) and becomes highly expressed in the human and murine T cells upon allo- activation making it an ideal target for allo-immunity. Hence, in collaboration with Dr. Gang Lin, we designed and synthesized a highly selective inhibitor of the LMP7 subunit of the i-20S that revealed an unexpected role for the LMP7 in T cell exhaustion through specific degradation of the JAK-STAT5 pathway and in promoting murine heart allograft survival across major histocompatibility complex (Esilida et al, PNAS, Dec 2016). While targeting c-20S is hindered by significant off target side effects, i-20S is an ideal target for allo-immunity as it's enriched in immunocytes upon activation. Our hypothesis is that during alloimmune T cell activation, the immunoproteasome subunit LMP7 is upregulated, promoting degradation of phosphorylated STAT5 (pSTAT5) through its protease activity to protect the antigen-reacting effector T cells from immune regulation and exhaustion. Hence, inhibiting the immunoproteasome subunit LMP7 increases pSTAT5, induces T cell exhaustion and Fas/FasL apoptosis leading to clonal deletion, and promotes allograft survival without toxic off target effects. To test our hypothesis, we will use selective inhibitors generated in collaboration with Dr. Gang Lin in an antigen specific transplant model (OTI, OTII, OVA) and will track the allo-reactive T cell clones using a novel non-invasive luciferase technique in vivo. We also generated knockouts of LMP7 on the OTI and OTII backgrounds and a conditional knockout of LMP7 selectively in CD4 and CD8 T cells. We will use molecular techniques to identify how the LMP7 degrade STAT5 and if STAT5 activation leads to T cell apoptosis and clonal deletion. Finally, to generate preclinical data, we will test our therapeutic strategy on the human immune system using a humanized mouse model transplanted with human islets.